Generally, metal-air cells comprise a metal-based anode and an air cathode as electrochemical active components separated from one another by an ion-conductive electrolyte. During discharge, oxygen is reduced at the air cathode with a gain of electrons. Hydroxide ions develop which can move to the anode via the electrolyte. There, a metal is oxidized with a loss of electrons. The developing metal ions react with the hydroxide ions.
Both primary and secondary metal-air cells have been developed. A secondary metal-air cell can be recharged by applying a voltage between anode and cathode and reversing the electrochemical reaction described above. Oxygen is released during this process.
The most popular example of a metal-air cell is the zinc-air cell. Button cells are particularly used as batteries for hearing aids.
Button cells involve, as generally known, cells having a height-diameter-ratio of less than 1. They usually comprise a liquid-tight casing composed of a bowl-shaped cell cup, a bowl-shaped cell lid and a seal. Cell cup and cell lid usually comprise in each case a bottom, a circumferential shell, a transition zone connecting the bottom and the shell, and a terminal cutting edge.
In a preferable configuration, the bottom of the cell cup and the bottom of the cell lid are planar and preferably circular, where necessary also oval. The shell of the cell cup and the shell of the cell lid can preferably be described as ring-shaped segments of a hollow cylinder having a circular or oval cross-section. Generally, the shells of the cell cup and cell lid are oriented orthogonally in relation to the corresponding bottoms.
The above-mentioned transition zones of the cell cup and cell lid preferably comprise those parts of the cell cup and cell lid outside of the plane of the respective bottom, but that are not yet part of the corresponding shell. The transition zones can be configured rounded-off, for example, in the shape of a shoulder or they may also have the form of a sharp edge.
Cell cups and cell lids are preferably manufactured from metallic materials such as nickel-coated steel or metal sheets. Furthermore, in particular also tri-metals are suitable, for example, having the sequence nickel, steel and cupper (from outside to inside).
When assembling a button cell casing, the cell lid with cutting edge ahead is inserted into the cell cup. The two parts are separated spatially and electrically from one another in the resulting casing by the above-mentioned seal which hence not only assumes sealing functions, but also insulating functions. Furthermore, the bottoms of the cell cup and cell lid are arranged parallel to one another in the resulting casing. The distance between the bottoms defines the height of the button cell. A straight line connecting the center points of the bottoms defines the reference axis (axial direction) along which insertion of the cell lid into the cell cup is effected when assembling the button cell.
For example, the cell cup and cell lid can be manufactured from nickel-coated deep-drawn metal sheets as punch-drawn parts. Usually, the cell cup forms the positive terminal and the cell lid forms the negative terminal. The liquid-tight sealing of such cells is often achieved by crimping the edge of the cell cup.
Metal-air cells have a relatively high energy density because the demand for oxygen at the cathode can be covered by atmospheric oxygen from the surroundings. Correspondingly, however, targeted measures are required to supply the cathode with oxygen from the surrounding air during the discharge process. And, vice versa, oxygen developing at the air cathode is to be lead off during the charging process. For this purpose, the casings of metal-air cells are systematically provided with inlet or outlet openings, respectively. Generally, this is effected by punching holes into the casings. Within the casings, the fine diffusion of the entering atmospheric oxygen is usually effected via suitable membranes or filters. Thus, the casing of zinc-air button cells is generally manufactured by use of a cell cup with air inlet holes punched into the bottom thereof.
For the manufacture of zinc-air button cells, a filter paper (or any other suitable, micro-porous layer) is inserted into such a cell cup to cover the bottom of the cup and the air inlet holes punched therein. Within the cell, the filter paper serves as a means for fine diffusion of atmospheric oxygen entering via the air inlet holes.
U.S. Pat. No. 4,118,544 discloses a button cell having such a means for air diffusion. In the case of the button cell described in U.S. '544, entry of air into the cell is regulated by a micro-porous layer or by the size of the air inlet holes. The size of the pores of the micro-porous layer or of the air inlet holes restricts the electric power of the cell. However, the cells shall temporarily also be capable of discharging higher currents. This is possible since in the cell, functionally analogous to the aforementioned filter paper, a layer for air diffusion (air diffusion layer) is provided where a certain amount of oxygen is stored.
Subsequently, the air cathode is laid onto the filter paper where the reduction of atmospheric oxygen can be effected. In turn, the cathode is covered with a planar separator forming the boundary layer between the cathode zone and the anode zone in the cell. Generally, such a pre-assembled cup part is combined with a bowl-shaped cell cup filled with zinc powder or any other suitable, reducible metal as anode material as well as with an electrolyte and on whose outer side a ring-shaped plastic seal is applied. The latter is inserted into the cell cup so that the plastic seal bears between the two casing parts. The cell is sealed in a liquid-tight manner by crimping the terminal edge of the cell cup over the inserted cell lid.
It could thus be helpful to provide a metal-air button cell having a simplified structure and an improved capacity.